Binary optical differential-phase-shift-keying (DBPSK) modulation has received considerable attention by the free-space optical (FSO) communications community and the telecommunications (“telecom”) industry. The increased attention is due to DBPSK's increased sensitivity over commonly used on-off-keying and reduced peak power which mitigates nonlinear effects. DBPSK's utility has been established with many long-haul fiber-optic experiments demonstrating multiple Tbit/s over ˜10,000 km fiber spans with hundreds of WDM-DBPSK channels (λs).
Another related modulation format, differential-quadrature-phase-shift-keying (DQPSK) modulation, has also attracted significant interest in the telecom industry due to its improved spectral efficiency, which reduces the impact of dispersion on fiber-optic links. Conventional DBPSK and DQPSK demodulators utilize delay-line interferometers (DIs) to perform demodulation.
It should be noted that while DPSK is often used to refer to binary-DPSK or DBPSK, the term used herein refers primarily to generic differentially encoded phase shift keyed modulation which includes DBPSK, differential-quadrature-phase-shift-keying (DQPSK), and M-ary-differential-phase-shift-keying (M-DPSK), in which the modulated information is typically encoded in the relative phase difference between consecutive optical pulses. Thus, demodulation of DPSK optical signals requires a differential phase comparison, which we refer to herein as differential demodulation.
FIG. 1A shows a typical DBPSK delay-line interferometer (DI) based demodulator 101 used in an optically preamplified receiver 100 for a single channel. In operation, a received DPSK signal 103 is first amplified via an optical amplifier 105. The amplified signal is then demodulated with use of the DBPSK demodulator 101. The amplified signal may first pass through a bandpass filter 107 and is thereafter converted to an intensity-keyed signal via a delay line interferometer (DI) 109, which may be driven by control electronics 111. The time-domain transfer function of the filter 107 is given by its impulse response h(t) which corresponds to the frequency response H(f).
Within the DI 109, the amplified signal may be split into two beams (not shown) of approximately equal intensity on two paths (not shown). One of the two beams is delayed in time by an optical path difference that introduces a relative time delay (τ) typically corresponding to one bit (τbit). The beams in the two paths may thereafter be coherently recombined to interfere with each other either constructively or destructively depending on the relative phase difference, so that the recombined signal is directed to one of two output arms a or b.
The power in the a and b output arms of the DI 109, Pa and Pb, respectively, is converted to a difference photocurrent isig via balanced detection 113. Note that the balanced detection operation can be performed using a balanced photo-detector pair 115 as shown in FIG. 1A. Alternatively, the difference photocurrent conversion may be performed using individual photo-detectors followed by differential electronics.
DQPSK can be demodulated in a similar manner as shown in FIG. 1B, with addition of a splitter 116 that breaks the optical signal into two approximately equal portions so that the in-phase (I) and quadrature (Q) components of the DQPSK signal may be demodulated separately with two similar DIs 109i and 109q, respectively. Typically the DIs are offset biased at ±π/4 in order to simplify the detection electronics and logical mapping 130 needed to convert the demodulated optical signal to data.
FIG. 2 illustrates a detailed view of the delay-line interferometer 109, as illustrated in FIG. 1A, implemented using a Mach-Zehnder interferometer. Alternatively, DIs can be constructed with other designs such as a Michelson interferometer geometry to achieve the same transfer function and ability to demodulate DPSK signals. The Mach-Zehnder DI 109 typically has two input ports 110 and 203 and two output ports a and b. An optical signal enters one of the two input ports, e.g., the primary input 110. The secondary or spare input port 203 is not typically used, but may be used to couple in a pilot tone that may be used to assist in stabilizing the interferometer. Input coupler 205 of the DI 109 splits the incoming optical signal into two, preferably equal-power signals onto paths 202 and 204. The optical lengths of the two arms 202 and 204 may be unequal, resulting in a relative time delay that is typically about one bit period (or the symbol period for M>2) as noted above. Regardless of the interferometer type, the relative delay τ between the arms causes the present optical signal s(t) to interfere with a delayed signal s(t−τ). Depending on the relative phase of the differentially encoded data, if the interferometer is biased appropriately, the two signals constructively interfere after recombining at coupler 207, directing all of the output power to one of the two output ports a or b, thus performing the differential phase comparison needed for demodulation. Simple thresholding circuitry may subsequently be used to convert the difference signal into logical data.